1. Field of the Invention
The invention relates to a receiver comprising an input for receiving an antenna signal, an output for supplying an audio signal, first and second mixing means for mixing the antenna signal with a first and second oscillation signal to obtain an in-phase and quadrature signal, filter means for filtering the in-phase and quadrature signal, demodulation means for demodulating the filtered quadrature signals and an oscillator for supplying the first and second oscillation signal to the first and second mixing means.
The invention further relates to a method for tuning a tuner to a predetermined frequency.
2. Description of the Related Art
Receivers are known in the art. A conventional receiver architecture requires external components for RF/image selectivity, IF selectivity and demodulation. To facilitate full integration of receivers, numerous novel architectures have been proposed. For a broadcast network where little or no information is present at the carrier frequency, for example, GSM or DAB, the zero-IF architecture is a good solution. However, broadcast systems with much information at the carrier frequency, for example, standard AM/FM broadcast cannot tolerate the DC consequences of the conversion to zero frequency.
Therefore, the low IF architecture has been proposed to solve this problem. However, converting the signal to a very low IF has the drawback that no image rejection can be carried out prior to this frequency conversion. The remedy to this is to use quadrature signal processing as is well known in the art. The limiting aspect of the quadrature low-IF architecture is that it delivers integrated image rejection based on matching of parameters, such as, gain and phase, of the quadrature channels. In practice, the matching of components in an integrated circuit limits this to about 40 dB. Some applications, for example, the mono-portable FM radio, can accept this limitation. But other applications, such as, FM stereo for portable and car application, require at least 60 dB of image rejection. To solve this matching problem, a number of correction algorithms have been developed. Unfortunately, these algorithms are only suitable for digital realization. The extra cost involved in digitalization make these solutions out of reach for cheap portable radio type of applications, for example.
European Patent Application No. EP-B-0 496 621, corresponding to U.S. Pat. 5,230,099, discloses a receiver having a number of correction algorithms. However, this is an expensive solution, and certainly not suitable for cheaper applications, such as portable radio receivers.
An object of the invention is to provide a receiver having additional effective image rejection suitable for all receiver realizations with very little additional complexity. This is achieved with a receiver according to the invention, characterized in that the receiver comprises measuring means for measuring a level of disturbance at a first and second predetermined frequency distance from the chosen frequency, comparing means for comparing the measured first and second level of disturbance, choosing means for choosing the frequency band with the lowest level of disturbance, and a control means for controlling the oscillator.
The idea of the invention is to position, at will, the xe2x80x9cweakxe2x80x9d image point at the side of the wanted channel with the lowest unwanted RF signal strength. In this way, also, while the actual image rejection of the circuit has not necessarily been increased, the actual perceived image rejection is clearly improved.